1. Field of the Invention
The present invention relates to an electrically actuatable disc brake that is capable of converting a rotary motion of an electric motor to a linear motion so as to press brake pads against a disc rotor to thereby generate a braking force.
2. Description of the Related Art
Conventionally, there has been known an electrically actuatable disc brake apparatus in which a rotary motion of an electric motor is converted to a linear motion of a piston by using a rotary-linear motion converting mechanism such as a ball screw mechanism, a ball ramp mechanism, etc., so as to allow the piston to press brake pads against a disc rotor to thereby generate a braking force. In the electrically actuatable disc brake, a driver's pedaling force that is applied to a brake pedal (or an amount of displacement of the brake pedal) is detected by a sensor and, based on the detected value, a controller controls the rotation of the electric motor to thereby obtain a desired braking force.
In order to amplify torque of an electric motor of an electrically actuatable disc brake, various proposals have been made with respect to an electrically actuatable disc brake in which a reduction mechanism is provided between an electric motor and a rotary-linear motion converting mechanism. For example, Unexamined Japanese Patent Application Public Disclosure No. 2001-263395 describes an electrically actuatable disc brake comprising a differential reduction mechanism in which planetary gears and an Oldham mechanism are used in combination.
In this electrically actuatable disc brake, the differential reduction mechanism is arranged such that an eccentric plate is rotatably attached to an eccentric shaft that is connected to a rotor of a motor, fixed pins are inserted through openings formed in the eccentric plate, and external teeth of the eccentric plate are meshed with internal teeth of a rotary disc of a ball ramp mechanism. With this arrangement, when the eccentric shaft is rotated, the eccentric plate performs an orbital motion without rotating on its center axis, thereby operating the rotary disc of the ball ramp mechanism at a predetermined reduction ratio.
The aforementioned Japanese patent application also describes a differential reduction mechanism comprising a cycloid ball mechanism and an Oldham mechanism using annular grooves and balls. This differential reduction mechanism exerts the same working effects as the above-described differential reduction mechanism.
However, the electrically actuatable disc brake described in the aforementioned patent application has the following problems. That is, the eccentric plate and pins forming the Oldham mechanism are costly to manufacture due to the need to achieve high machining accuracy. Further, although high strength is required to withstand a high load during braking, it is difficult to increase the strength of the Oldham mechanism due to its structure.
Similarly, in the differential reduction mechanism comprising a cycloid ball mechanism and an Oldham mechanism, high machining accuracy is required. In particular, the requirements for axial dimensional accuracy are especially strict, which involves problems in practical terms.
The present invention has been made in view of the above-described problems with the conventional apparatuses. It is therefore an object of the present invention to provide an electrically actuatable disc brake comprising a reduction mechanism having a simple structure and sufficient strength and which is capable of providing a desired reduction ratio.